vertexcodec: Fix encoding for vertex_count = 0
vertexcodec: Fix encoding for vertex_count = 0

It's not clear if this is really useful, but just in case it is this
fixes the crash when encoding an empty vertex buffer; last vertex will
be zeroed out in this case. Decoder already supports vertex_count = 0
without changes.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer